IL-5 is expressed in eosinophils, NK cells, TC2 CD8+ T cells, mast cells, CD45+ CD4+ T cells, gamma delta T cells and IL-1 beta activated endothelial cells. IL-5 acts as a growth and differentiation factor for both B cells and eosinophils. Relative to B cells, IL-5 appears to induce the differentiation of activated conventional B-2 cells into Ig-secreting cells. In addition, it induces the growth of B-1 progenitors as well as IgM production by B-1 cells.IL-5 appears to perform a number of functions on eosinophils. These include the down modulation of Mac-1,the upregulation of receptors for IgA and IgG,the stimulation of lipid mediator (leukotriene C4 and PAF) secretion and the induction of granule release.IL-5 also promotes the growth and differentiation of eosinophils.